Field of the invention
The present invention generally relates to a resistive sheet thermal transfer printer, and more particularly to a resistive sheet thermal transfer printer in which unevenness of the density of a line image formed of a plurality of dots can be prevented. The unevenness of the density of the line image is referred to as a multi-dot density unevenness.
A description will now be given of a conventional resistive sheet thermal transfer printer with reference to FIGS.1 and 2.
Referring to FIG.1, an ink sheet 501 is put upon a recording sheet 502. The ink sheet 501 has a resistance layer 501a, a conductive layer 501b and an ink dyes layer 501c. The conductive layer 501b is sandwiched between the resistance layer 501a and the ink dyes layer 501c, and the ink dyes layer 501c is in contact with the recording sheet 502. The resistance layer 501a is made, for example, of an Aramid film including carbon grains. The conductive layer 501b is made, for example, of aluminum. A common electrode 503 and a plurality of recording electrodes 504 arranged in a line are arranged at predetermined interval on the resistance layer 501a of the ink sheet 501. The common electrode 503 and the recording electrodes 504 are pressed against the resistance layer 501a.
When a voltage is applied across the common electrode 503 and each of the recording electrodes 504, an electric current flows through a first part of the resistance layer 501a corresponding to the common electrode 503, the conductive layer 501b between the common electrode 503 and each of the recording electrodes 504, and a second part of the resistance layer 501a corresponding to each of the recording electrodes 504, as shown by arrows in FIG.1. In this case, when the electric current flows through the first and second part of the resistance layer 501a, Joule heat is generated from each of the first and second parts of the resistance layer 501a. An amount of Joule heat generated in the resistance layer 501a is proportional to the square of electric current density therein. Thus, as the end surface of each of the recording electrodes 504 is smaller than that of the common electrode 503, the generation of Joule heat is concentrated in the second part of the resistance layer 501a corresponding to each of the recording electrodes 504 (an area shown by a slanted lines in FIG.1). The ink in the ink dyes layer 501c, positioned under each of the recording electrodes 504, is fused and sublimated due to the Joule heat in the second part of the resistance layer 501a, so that ink corresponding to each of the recording electrodes is transferred to the recording sheet 502.
In the above conventional resistive sheet thermal transfer printer, when the voltage is simultaneously supplied to successive some of the electrodes 504, the following problem occurs.
For example, when the voltage is simultaneously supplied to three of m recording electrodes 504, second (2), third (3) and fourth (4) recording electrodes 504, as shown in FIG.2, an additional current flows into end positioned recording electrodes, such as the second and fourth recording electrodes (2) and (4), from a periphery thereof. Thus, in this case, the amount of current flowing into the third recording electrode (3) between the second and fourth electrodes (2) and (4) is less than the amount of current flowing into the end positioned recording electrodes, the second and fourth recording electrodes (2) and (4) in this case. That is, the amount of heat generated in the resistance layer corresponding to the third recording electrode (3) is less than the amount of heat generated therein corresponding to the end positioned recording electrode. As a result, unevenness of the density occurs in the image formed on the recording sheet 502.
To eliminate the above problem, conventionally, "ELECTRIC INK TRANSFER RECORDING METHOD USING MULTI-STYLUS" has been proposed in THE JOURNAL OF THE INSTITUTE OF IMAGE ELECTRONICS ENGINEERS OF JAPAN 16, 1, (1987). In this method, one line is divided into a plurality of blocks, and the voltage is not simultaneously supplied to adjacent recording electrodes.
However, in the above method, as one line image is printed through printing a divided plurality of blocks, the time required for printing one line increases. Thus, the printing speed is decreased.